This invention relates generally to texturing of surfaces for improving the bonding strength between two surfaces to be attached together and also to an exemplification thereof in connection with semiconductor die or chip packages for encapsulating a die or chip on a chip carrier with appropriate leadouts for electrical connection, for example, to a printed circuit board (PCB). In particular, the texture indentation of the surface of die attachment pads in such chip carriers is provided to enhance the bonding strength between the die or chip and the die attachment pad surface as well as between the die attachment pad surface and the encapsulating medium employed in package molding applications.
While this invention has particular application in texturing of die attachment pads in die or chip carriers in semiconductor or IC packages, it will be appreciated by those skilled in the art that it also has many other applications where surface-to-surface attachment can be enhanced by texturing one or both of the surfaces to be attached. Such surfaces may be attached by means of an adhesive, solder, surface melting, laser welding or attachment, or other possible methods of securement of the surfaces. Other examples, among many such possibilities, are circuit components, including capacitors and resistors, to a surface or circuit board; and attachment or lamination of layered disk surfaces or other layered film surfaces.
As is known in the art, chip carriers, such as in the form of a metal lead frame or plastic carrier, such as a ball grid array (BGA) type package or equivalent overmolded or glob top type chip carriers, provide support and encapsulation of a semiconductor die or chip with appropriate leadouts from the chip for electrical connection to another electronic component, such as a PCB. In these packaged chip carriers, a die attachment pad is provided for supporting and attachment of the die or chip to the carrier usually by means of an adhesive, which is generally of thermally and electrically conductive material. After appropriate wire bonding has been accomplished with leadouts from the chip, the chip carrier is then encapsulated with an epoxy, resin or other molding material to protect the die or chip from the environment, such as from penetration of moisture and atmospheric corrosion, as well as protecting the wire bonded chip from damage. However, due to differences in the coefficient of thermal expansion (CTE) of the materials comprising the chip, the adhesive and the die attachment pad, the chip may become dislodged, detached or delaminated from the die attachment pad due to subsequent high temperature processing, such as the application of the encapsulation material, or high temperature curing of the die attaching adhesive, or from baking the package to remove moisture from the package that has penetrated the encapsulation material, or due to solder reflow applications. In addition to the package being subjected to such high temperature processing, the high temperatures also cause moisture contained within the package as well as other gases that may be present within the package, or possibly generated from the adhesive itself, to rapidly expand causing chip detachment. This particular type of detachment or delamination of the chip from is pad has been referred to as the xe2x80x9cpopcornxe2x80x9d effect.
The problem of xe2x80x9cpopcornxe2x80x9d detachment is also discussed in U.S. Pat. No. 5,696,666 to Miles et al. The solution is to eliminate altogether the die attachment pad while providing for lower profile chip carrier package. However, the supporting strength of the carrier pad-to-chip bonding is lost so that minimal package support is left to the side surfaces of the die which may result in die detachment from the lead frame during the subsequent high temperature process to which the package is subjected to, such as solder reflow operations. On the other hand, U.S. Pat. No. 5,708,294 provides a lead frame with a plurality of diagonal slots formed in the die attachment pad for the purpose of enhancing resin flow during the chip carrier encapsulation process. The slots in the die attachment pad enable enabling better resin flow along the bottom of the pad to avoid resin voids and die pad shifting within the lead frame network. However, there is no accommodation for enhancing the bonding strength between the chip and its attachment pad but rather accommodation for the flow of encapsulating resin along the bottom surface of the die attachment pad.
What is needed is an improvement for enhancing the bonding strength between the die or chip and its die attachment pad.
It is an object of this invention to enhance the bonding strength between surfaces of devices or structures to be attached together.
It is another object of this invention to provide enhancement strength between a die or chip and its die attachment pad.
According to this invention, the foregoing problems are solved by enhancing the bonding strength between surfaces that are to be secured, for example, by lamination or an adhesive, through the provision in at least one of the surfaces a plurality of indentations using a laser output, such as developed from a fiber gain medium texturing system or a YAG laser system. The fiber gain medium is preferred because such a system is modulated by means of the seed or pump laser diode source which is the origin of the system texturing output which provides versatility over YAG systems in the control of pulse power, duration and frequency, which is not altogether possible in YAG laser systems. The particular application for illustration of this invention comprises the texturing of die attachment pad surfaces in die or chip carriers used in semiconductor or IC packages. However, as previously indicated, the utility of this invention has application in other structures where improved surface-to-surface bonding strength is desired.
The bonding strength between a die or chip and its die attachment pad of a chip carrier or lead frame by is enhanced texturing the surface of the die attachment pad using a laser texturing system that can be easily pulsed to provide high peak pulses to conveniently ablate the surface of the die attachment pad. Such a laser texturing system in the form of a fiber gain medium, seeded or pumped with a modulated laser diode source, conveniently and efficiently provides for variations in the surface size and depth of the indentations. The textured indentations comprise a plurality of dimples or grooves or combinations of different sizes of dimples and/or grooves. While indentations may be formed in the surface of the die attachment pad by means of a stamping or etching step, these type of processing steps are more physically intrusive on the chip carrier in that they can change the alignment of the pad relative to the lead frame framework and otherwise require a processing step that requires die handing of the carrier which may directly effect its integrity. The use of a modulated laser beam or pulsed laser output from a fiber amplifying medium having a master oscillator comprising a pump or seed semiconductor laser diode modulated source provides a controlled pulse output that is significantly less intrusive, less time consuming, and provides a processing step that is not major step in the manufacture and preparation of the chip carrier.
Thus, in accordance with this invention, a semiconductor die or chip carrier comprising a die attachment pad having a die attachment pad surface for receiving a semiconductor chip with the chip bonded to the die attachment pad with an adhesive provides for enhancement of the between the chip and the die attachment pad by forming a plurality of indentations in the die attachment pad surface with a modulated laser output amplified with a fiber gain medium, with the origin of the amplified output provided from a modulated semiconductor laser seed or pump source.
As used herein relative to a master oscillator, such as a seed or pump laser diode, the term xe2x80x9cmodulatedxe2x80x9d means a frequency modulated light output to produce a pulsed output at a repetitive rate, and a light output that is turn ON and OFF according to the requirements for texturing, such as the formation of indentations in the form of grooves wherein the output is turn ON and operated cw or at a pulsed rate as the output is scanned at the surface to produce the longitudinal extent of the groove, after which it is turn OFF. This is referred to as article marking or texturing by means of a fiber amplifying system that provides power-on-demand.